ABSTRACT Maturation of fetal organs to support life after birth is a significantly vital process. However, in most cases, this process is poorly investigated/understood. In the lung, failure in maturation is a significant health problem. In preterm infants, it can cause Respiratory Distress Syndrome (RDS) and BronchoPulmonary Dysplasia (BPD). We have found that loss-of-function for a non-canonical WNT ligand, WNT5a inhibits lung maturation. Two key defects include inhibition of alveolar type 1 epithelial cell (AEC1) differentiation and alveolar myofibroblast (AMF) differentiation. Importantly, the converse is also true; Wnt5a gain-of-function promotes lung maturation & AEC1 differentiation. In additional preliminary studies, we show that loss-of- function of WNT5a receptors, ROR1 and ROR2, results in similar defects in lung maturation as in WNT5a loss-of-function. WNT5a activates Calcium/calmodulin-dependent protein kinase II (CaMKII) an important cytoplasmic signal transduction molecule. Inhibiting CaMKII also disrupts lung maturation. Based on the collective preliminary findings, we propose the following hypothesis: HYPOTHESIS: WNT5a-ROR signaling is a key regulator of lung maturation. This pathway controls the structural, cellular and molecular steps that ultimately contribute to the formation of functional lungs. In this application, we will focus on AEC1 and AMF differentiation, as two key cellular mechanisms controlled by WNT5a during lung maturation. Multiple tissue specific, conditional in vivo models that target Wnt5a and Rors in AEC and AMF during lung maturation will be used to determine their cell-type specific functions in saccular and alveolar stages. The studies will also determine the role of CaMKII as WNT5a mediator. We propose the following Specific Aims. Specific Aim 1. To Determine the Function of WNT5a Signaling in AEC During Lung Maturation. Specific Aim 2. To Determine the Function of WNT5a Signaling in AMF During Lung Maturation. Specific Aim 3. To Determine the Role of CaMKII as a Downstream Intracellular Mediator of WNT5a Signaling in Lung Maturation. Health Relevance: Lung maturation is an existential requirement for surviving after birth. If successful, the results of this research will likely have a meaningful impact on understanding the mechanisms underlying lung maturation, which may help in development of more effective & novel therapies for lung diseases in premature infants. Determining the role of WNT5a, RORs & CaMKII in lung maturation could further suggest therapeutic targets at different levels of signal transduction & may allow for more selective modulation of this process.